Flow-control system for rotaryvane compressors for conditioning air



Mach'Z, 1953 B. s. MASSEY ET AL 2,632,307

FLOW-CONTROL SYSTEM FOR ROTARY-WINE OOMPREssORs FOR CONDITIONING AIR 4 Sheets-Sheel 1 Filed Oct. 50. 1950 March 24, 1953 B. s. MAssr-:Y .ET AL 2,532,307

FLOW-CONTROL SYSTEM FOR ROTARY-VANI cOMPREssORs FOR CONDITIONING AIR Filed Oct. 50, 1950 4 Sheets-Sheet 2 .12. L. 1u/NN 2. ALLE/v C. WAQA/E March 24, 1953 B. s. MAssEY ET AL 2,632,307

mow-CONTROL SYSTEM FOR ROTARY-vm;

COMPRESSORS FOR CONDITIONING AIR Filed Oct. 50, 1950 4 Sheets-Sheet 3 f SQ I, I@ s-s l@ @S5 19 T $2 "T w/ Q ik@ ik@ 1&1 NMMOMWMH March 24, 1953 B. s. MAssEY ET Al. 2,632,307

FLOW-CONTROL SYSTEM FOR ROTARY-MNE COMPRESSORS FOR CONDITIONING AIR 4 Sheets-Sheet 4 Filed Oct. 30, 1950 /m/fA/mes 6. .si M4555 y J 2. ALL E 5. C. M42/V56 mmlwo'm MM5 1 Patented Mar. 24, 1953 UNITED STATES PATENT OFFICE FLOVV-CNTROL SYSTEM FOR ROTARY- VANE COMPRESSORS' FOR CONDI- TIONING AIR Application October 30, 1950, SerialNo. 192,846 In Great Britain November 1,A 1949s 21 Claims.

This' invention concerns controlv systems for regulating the volume-flow output of rotary-vane compressors, by which are meant centrifugal and axial flow compressors, and relatesl more particularly to control systems for cabin-pressurising plant of which the compressor forms a part.

With compressors of the centrifugal and axial flow type it is found that the compression ratio falls and the compressor becomes less efiicient as a compressor as the volume flow increases. As thevolume ow is diminished a higher compression ratio is obtained until a point is reached in which operation of the machine becomes unstable with periodic fluctuations of flow and delivery pressure. This condition is generally known as surge To obtain a high eiciency it isdesirableto operate the compressor under conditions approacl'iingv surge but sufliciently far away from these conditionstg allow for unavoidable variation in the volume flaw.

The object of the present invention is broadly' to provide a control system such that the volume flow output of the compressor is automatically regulated and maintained. approximately constan-t, With such automatic control the compressor may be operated close to the surge condition while ensuring that the condition isl not reached,

Another object of the present. invention is to provide a controlsystem for a, cabin pressurising plant whichwill ensure that theA amount of fresh air; which.Y isV supplied for the passengers and/or crew does not fall-v below a predetermined. mini-` mumand does not rise toa value greater than is reasonably necessary to avoid surging; in the compressor', while keeping" a selected pressure in the cabin up. to a predetermined-altitude and running such airconditioning apparatus as may be necessary.

At further object of'- the present invention is to l maintain the temperature and humidity in the cabin withinA specied limits over a wide range of ambient conditions.

According to the present invention a control system for regulating. the output of a centrifugalor axial flow compressor comprises means to vary the mass flow of air passingl through the compressor in direct proportion to the absolutedelivery pressure of the compressor andA in inverse'. proportion toA the square root of the absolute temperature ofl the delivered air.

According to a feature of the present, invention. a control system for regulating the output of a centrifugal or axial iiow compressor comprises means: to regulate-the quantity of air entering the compressor, a Venturi-system to which the compressor delivers, means responsive to theabsolute delivery pressure ofthe compressor, means responsive to the differential pressure between the inletY and throat ofi the Venturisystem and means controlled by said" pressure-responsivemeans to adjust the air-regulator means to vary the quantity of airpassing through the compressor insuch: a way that the ratio of the Venturi differential pressure to the compressorl delivery pressure is maintained constant.

According to another feature of the present invention a control system for regulating the output ofl arotary-vane-l compressor comprises` a throttle valve on the intake side of the compressor, a Venturi system or the output side of the compressor, a capsule responsive to the absolute delivery pressurer of the compressor, a capsule responsive to the: differential pressure between the inletr and' the throat of the venturi, a servo system for adjusting the throttle valve and means applying the capsule adjustments to the: servosystemwhereby they throttle valve is adjusted to vary the quantity of air passing. through the compressor in such away that the ratio of the Venturi diiferential pressure toV the compressor delivery pressure is maintained constant.

Af practical. application of the-present,A invention will. now be described, by way ofexample only,

withV reference; to the accompanying drawingswhich show a cabin pressur-sing plant embodying' the present invention. In the drawings-n Figure 1 is a block diagram illustrating the plant as: a whole.

Figure 2 shows the principal parts of the 110W control system,v partly diagrammatically and with parts cut away to show internal construction.

Figure,A 3/ is a longitudinal section through. a uid. pressure actuator an d1,servo device shown in Figure 2.

l'igurestv to 8 inclusive are sections'ta'ken along lines 4*-4 to* 8;-8' inclusive in Figure 3.

Fig-ure 9' is a diagram illustrating the characteristics of the compressor;

Referring to Figure 1, an aircraft cabin l is supplied with air through` a non-return' valve 2 by a plant arranged in the wing of` the aircraft, the air passing along l distribution ducting 3', possibly withv the addition of recirculated: air; to

variousr parts of the1 cabin as required, and then beingreturnedl to thev atmosphere at least partly through a discharge valve 4 which is controlled by a: pressure sensitive device. 5 of known kind which permitsy the pressure in thev cabin to be maintained at a desired value or varied according to a desired law in relation to changes of altitude of the aircraft.

The air supply plant comprises a compressor 6 of the rotary vane type (centrifugal or axial flow) driven from an engine 'I of the aircraft through a variable speed gear 8, shown as a two speed gear, though a greaterrnumber of speeds may be provided if necessitated by the design and operational characteristics of the plant. The variable speed gear comprises an actuator Y9 for selecting ,neutral or engaged and an actuator I for selecting low speed or high speed. These actuators may, for example, be solenoids operating control valves to admit fluid under pressure to hydraulically operated clutches to carry out the required gear changes. The actuator 9 is connected through a switch I I for starting and stopping the plant to a mains supply at I2, ,while the actuator i is connected through the plant switch I I and a speed control switch I3 to the mains supply at l2. The speed control switch i3 is operated by an evacuated capsule I 4 responsive to atmospheric pressure, the arrangement being sueh that at low altitudes the variable speed gear is maintained in the low speed setting, and that when a predetermined altitude is reached a change is made into a higher speed. In the case of a two speed drive for a plant designed to operate up to altitudes of the order of 40,000 ft. above sea level the change may be made, for example, at an altitude of about 22,000 ft. The switch i3 includes a trip or equivalent mechanism whereby contact is made for an upward speed change at an altitude somewhat greater than the mean change-speed altitude and is broken for a downward speed change at an altitude correspondingly below the mean changespeed altitude. This overlap range may be of the order of 1G00 ft. and obviates unstable running of the plant when the aircraft is flying at about the change-speed altitude.

The plant also includes the usual air conditioning devices, namely a cooler I5, heater I6, refrigerator IT and water separator I3. The

cooler I5 and the heater I5 are of the heat ex-l change type in which cold or hot air is passed in heat-exchange relationship with the air flowing to the cabin along the duct I9 from the compressor 6. Control of the cooler and heater is by means of valves 2li and 2I respectively in the cooling and heating air circuits. The refrigerator il' is of the free-running compressor-expander type with an interposed secondary cooler and is controlled by a valve 22 arrangedin a by- Vpass duct 23. The heater, cooler and refrigerator are controlled in progressive sequence by a variable datum thermostat 23 arranged in the cabin, where it may be set by an engineer cr steward, and adapted to operate the valves 20, 2I and 22 by means of an actuator 2T. As shown in Figure 2, this actuator may be a geared electric motorY Z8 driving a camshaft 29 carrying three cams 30, 3l and 32 pertaining to the heater, cooler and refrigerator control valves respectively. The refrigerator cam 32 operates the refrigerator by-pass valve 22 in the opening direction through the intermediary of a cam follower 33, shaft 36, gear segment 35, rotary vane servomotor 3S, shaft 3l and operating lever 33. The cooler valve 2G and the heater valve 2l may be operatedin a similar manner through servomotors, not shown on the drawing. rhe water separator I8 is arranged inl a ,by-pass duct 24 and is brought into action when required by a 4 V humidistat 39 arranged in the cabin and controlling the closing of a valve 25 provided in the main duct I9. I Y' Since the refrigerator Il is not required at altitudes exceeding 22,000 ft. (the mean changespeed altitude) it is preferred to provide an additional refrigerator by-pass valve Il@ which is held. open by an actuator 4I when the compressor speed control switch I3 is in the high speed setting.

The quantity of air passing through the compressor 6 is controlled in accordance with principles which will now be explained by a flow control device !32 acting on a throttle valve 53 in the compressor intake duct 4, on the refrigerator by-pass valve 22, and on a spill valve l5 arranged in a duct i6 branching from the main duct I9 between the heater I 6 and the refrigerator I 1.

Referring to Figure 9, the operating characteristics of a rotary vane compressor of the centrifugal or axial ow types may be represented by curves drawn using as non-dimensional coordinates compression ratio and an inlet ow function WVZ/Pz, in which:

W :Mass air flow p-er unit time.

P2=Absolute pressure at impeller intake (after any throttle valve that may be fitted).

T2=Absolute temperature at impeller intake.

Pas-:Absolute delivery presure.

All pressures and temperatures referred to in this specification, with the exception of the Venturi throat pressure PT referred to later, are total head values, i. e., they include the pressure and temperature equivalents of the velocity of the airstream.

On such a diagram a family of curves A, B, C may be drawn for each of which impeller tip speed divided by \/T2 is equal to a different constant value. It is found that such curves droop more and more steeply as the conditions of operation tend towards higher values of the inlet ow function, that is to say, the compression ratio falls and the machine becomes less and less efficient as a compressor as the flow increases. Working in the other direction, however, towards a smaller now and a higher compression ratio a point is reached in each curve in which the operation of the machine becomes unstable, periodic fluctuations of mass flow and delivery pressure occurring which may be sufficiently violent to cause mechanical failure. This condition is known as surge, and the curve D joining the surge points in the curves A, B, C, is termed the surge line. To obtain maximum eiciency itis clearly desirable to operate the compressor under conditions approaching the surge line but sufliciently far from it to allow for unavoidable variations. It is found that if a family of curves are plotted such that the outlet flow functions Wx//Ps-:a constant, these.

5a pei'ature rise through. the compressor varies directly as the squarev of the tip speedr It caribe shown, and. has been experimentally confirmed, that the law of a'- Venturi system such. as that'shownl in Figures 1 and 2 of the drawings is` of the following form;

Where P3 pressure at entry to venturi Pr throat pressure PD diierential pressure P3 P'r Wand. T3 are the low and temperature atent'ry' to the venturi.

If, therefore, a venturi. is tted at the blower delivery, asv shown in Figures 1 and 2, and some1 means of. keeping Pis/P3. constant is found, then Wx/Ts/Pr;v will also be llrep't constant as is desired in order to satisiy the equation of the operating' line E of Figure 9,

The meansy employed' for this purpose are shown in Figure 2 and. comprise an evacuated capsule 41 mounted in a closed boxV 48' connected by a conduit 49 to a pressure point' at the inlet to the venturi 5t, so that the capsule contracts in proportion to the delivery pressure P3. The moving end` of the capsule i1 is connected to one arm ofv a lever 5lA which is pivoted about an intermedi'- a-te point 52` and the other arm of which is connected to: one end of a capsule 534 the interioro whichis connected by means of a sliding sleeve 511 and :a conduitl 55 to the throat of the venturi 50. The other end of the capsule 53 is connectedy by means of a pivoted lever 5S to a huid pressure 11- reservoir open' outside the lands 58', 59, the arf rangement being such that movement ofv the valve' 5? in one direction orl the other from the equilibrium position allows pressure fluid to enter the motor 62 on one side'or the other' of its rotating vane, whiler the chamber on theother side ofu the rotating vane is connected to the reservoir. supposing now the deliveryv pressure P- remains constant', while the differential pressure 'Pny which is' the diierence between the pressures in the conduits d'9'k and 55, increases dueto increased airflow, it will be seenthat the capsule 41 and hence the levier 5i remain unafected while the' capsule 53 tends to collapsethus moving the valve: 5T towards the right. This movement initiates operation of' the throttle master vane moto-r- 62 in the direction` to closethe throttle 43 so that the ow falls. When the flow has fallen to its orig-L inal value the valve 51 willf have' returned t'o'. its equilibrium position, thus the conditionY PIJ/Ps: constant is satised in thisI case.

Suppose now the delivery pressure Ps increases', duefor' example to the refrigerator being brought into action, the capsule '11 will contract', causing.' the valve4 51- to move' to the lett and thereby initiating movement of the intake throttle val-ive 4.3 in the opening direction. The: resulting' increased ow will cause acorresponding increase: the differential pressure Ps-Pr So that the' diierential pressure capsule 53 contracts, moving the valve 51 back towards the right. Itwill therefore SLI be seen. that displacement ofthe valve 51 towards the right is directly proportional tov the differ entialv pressure Pn and inverselyT proportional to: the delivery pressure P3', and. that the system tends to return to a condition in which thev valvef 51 lies in, the. neutral (constant displacement)` position in relation to the oil supply and return conduits. In other words thev vcondition PD/Ps constant is. again satisfied. and the system can be made to operate along the. chosen operating vline E provided the constants of the system are suite ably chosen. TheseI constants are the flexibility of. the capsules 47 and. 53', the proportions which the. two arms of. the lever 5I bear to eachother,

and thev value of Pis/P3. produced. by tlrielv Venturi systgiA for the desired. vconstant value of. WVTs/P.

It has already been shownv that when the de* livery pressureP3 rises due to the refrigerator being brought into action, the inlet temperature, impeller tip speed and the. cabi-n pressure` being assumed constant, the air ow increases in pro-- portion. This feature is an advantage rather than otherwise'sinee i-t means that an equivalent cooling ei'ect can be'- obtained with a smaller reduction of temperature through the refrigerator.- Moreover suiilcient cool air is available to supply a system' of air jets under the control of individual passengers. If, as is usual, the cabin ventilation system includes means 'for mixing recirculated air with the fresh air from the supply plant, possibly at the rate of three times theV normal minimum ow of fresh air, it will be seen that,if the flow of recirculated air remains the' same, theproportional increase' in the total air in circulation isvv much smaller than that in the' fresh air.

Considering now the effect or changes` of comi presser speed, for example when during pressurising at ground level without refrigeration the engine speed is increased from cruise to take offA R. P. M., the" inlet'y temperature Tt* remains con` stant and the delivery'press'ure isv held constant by the `cabin discharge* valve, but the outlettern'- p'erature T3 will. increase, since the' temperature rise through the compressor varies as the square of the speed. In these circumstances the operating line equation WVTa/PFK shows that the' a'rfIow'W decreases since' Ts increases. A similar condition. arises. whenA the compressor drive changes from low to high gear at constant engine speedl.. The air supply plan-t must of course" be designed. to supply adequate air tothe cabin.un'-l der the minimum w condition.

It already been. mentioned that when the' aircraft is flying at anA altitude such that the' cabin differential pressure constitutesa substantial proportion of the compression the compressor is capable of'. producingr there may not be suffi.- cient reserve pressure available to runthe'refrig.- erator. to the required output. I-n such. a case the cabin thermostat 26` will. control the actuator 23 15o-'move thereirigerator cam 32' to its lowest position. in relation. to thev follower 33 so that the shafts 34and. 31 turn. in. the directions of the arrows .r and. y. respectively, which correspond. to closing. of the refrigerator ley-pass valve 22. rl-he deli-very pressure should. now rise to a value suicient to causev an airflow through` the system. satprying.; the operating une equation. wv/Peec..

and the ow control'. 4E. will` move the throttle valve.' 43f towards a more open position. in. an Ven` de'avour'to'l satisfy this demand. If the demand is not sati'sedf by' the: time the throttle valve 43 is fully open the operati-ng; condition. ofV l'.l1e.c'on'r.l

pressor will move towards the surge line D, with consequent danger to the plant. To avoid this possibility provision is made, and will presently be described, whereby the master flow control actuator 82, after causing full opening of the throttle valve 43, overrides the temperature-responsive control of the refrigerator by-pass valve and moves this latter towards the open position. 'Ihe demand of the cabin temperature control is thus made subordinate to that of the cabin pressure control.

Another case in which compressor surge might occur would be the partial or total blockage of the flow paths from the compressor to the cabin. For example such a blockage could occur in the case of an aircraft having two independent air supply plants if one compressor failed to change speed at the correct altitude and the aircraft continued to climb. When the low-speed ceiling of the blower running at low speed was reached, its non-return valve at entry to the cabin would close, because the other compressor running at higher speed would be able to maintain the correct cabin pressure, and the compressor running at low speed would go into surge. In such a case the ow control system would open the refrigerator bypass Valve 22 as described in 'the preceding paragraph, but this would not remove the blockage, and it is therefore necessary to open the spill valve 45 sufliciently to permit the required flow of air. The means by which these valve movements are carried out will now be described.

As will be seen from Figure 2 the ow control master actuator is arranged with its shaft in alignment with two rotary vane type servo-motors. The lservomotor 3B on the left operates the refrigerator by-pass valve 22 as already stated, while the servomotor 61 on the right operates the intake throttle valve 43 through gear segments 68, 69, shaft 18 and operating lever 1|. Referring to Figure 3, which is a longitudinal section through the assembly of servomotors, the casing 12 of the master actuator '52 comprises an internal oil conduit 13 which, as seen from Figure 4, forms a continuation of the pressure fluid conduit' 68 and opens through ports 14, into chambers 16, 11. In a similar manner, the pressure fluid conduit 6| is connected to a conduit 18 in the casing 12 which opens into chambers 19, 88. When therefore the flow control valve 51 moves towards the r right and fluid under pressure is admitted to the conduit 6|, such fluid passes into the chambers 19, 88, while the opposite chambers 15, 11 are connected through the conduits 60, 65 to the reservoir. The vane 8| of the actuator therefore turns in a clockwise direction as seen in Figure 4, or in the direction of the arrow y as seen in Figure 2. Similarly, movement of the valve 51 to the left causes rotation of the vane 8| in an anti-clockwise direction.

The shaft of the vane 8| has an extension 82 constituting a pilot valve lying within the vane 83 of the servomotor B1 which operates the intake throttle valve 43. As seen in Figures 3, 5 and 6 the pilot valve 82 comprises a circumferential groove 84 from which diametrically opposite longitudinal feed grooves 85 extend part way towards the end of the valve. Spaced between the feed grooves are also a pair of discharge grooves 86 extending from the end of the valve part way towards the cir-cumferential groove 84. The groove 84 communicates through ports 81 and a circumferential groove 88 in the shaft of the vane 83 of the servomotor 61 with a conduit 89 which is connected at 64 (Figure 2) to the source of fluid under pressure. As shown in Figure 6, the vane 83 is provided with four ports 98, 9|, 92, 93 which communicate with the four working chambers and are all shut off by the pilot valve 82 when the latter is in the relative position shown. If now the pilot valve is rotated in a clockwise direction as seen in Figure 6, fluid under pressure will pass from the grooves through the ports 9| and 93 and will turn the vane 83 until it is on-ce more in the same relative position with regard to the pilot valve. In other words, the vane 93 follows the movements of the pilot valve, and therefore of the master actuator vane 8| so long as its free movement is not prevented by engagement with the xed abutments 94 and 95. In Figure 4, movement of the master actuator vane 8| from the line a a to the line b b corresponds to the opening movement of the intake throttle valve 48, the valve being actually moved by the servo Vane 83' in moving from the line a.' a to the line b b. Further movement of the master actuator vane cannot be followed by the servo vane since it is stopped by the abutments 94 and 95.

The construction of the refrigerator by-pass valve servomotor 35 is similar to that of the servomotor 61 and requires no further explanation except to state that the pilot valve 86 carries a pinion 91 (Figures 2 and 3) meshing with the lgear segment 35 of the temperature control system and also a pair of dogs 98 by which it is in lost-motion connection with corresponding dogs 99 on the master actuator vane 8| (see Figure 7, which is drawn to a larger scale than the other sections).

The sections inFigures 4, 6, '1 and 8 are drawn to show the relation of the ports during full refrigeration, that is to say the throttle valve servomotor vane 83 is at the end of its working stroke in the throttle open position, while the refrigeration by-pass valve servomotor vane '|08 is at the end of its working stroke in the Valve closed position. The master flow control actuator vane 8| has passed through part only of its working stroke, which is greater than that of the vane 83, and has brought its dogs 99 just into contact with the dogs 98 on the refrigerator pilot valve 95. If, when in this setting, the operating line equation is not satisfied, the master vane 8| will rotate further in an anti-clockwise direction owing to the flow control valve 51 remaining displaced to the left in Figure 2. The dogs 99 will therefore rotate the pilot valve in the same direction, and the vane l 89 will follow and will cause opening of the refrigerator by-pass valve 22. If stable flowconditions can be established by partial opening of the lluy-pass valve, the system becomes stabilized in that setting but if this is not attained by the time the valve has fully opened, corre sponding to a line such asy c c in Figure 8, a pair of complementary cams |8| on the shaft 31 lcome into action through the intermediary of cam follower |02, gear segment |83, differential gear |84, shaft |85 and operating lever |86 to open the .spill valve 45. The travel of the vane |88 from the line c c to the line d d therefore corresponds to an extension of the movement of the refrigerator by-pass valve 22 beyond its fully open position and to the opening range of the spill valve 45.

This arrangement is represented on Figure l by on override connection |91. The opening movement of the spill valve continues only until the operating line equation is satisfied.

A manual control |88 in the cabin controls an actuator |85 geared to the differential |84 so that the spill valve 45 may .be deliberately set fully open if desired, for example when starting up the plant or when testing it at ground level.

We claim:

1. A control system for regulating the output of a centrifugal oraxial ow compressor com-- prising means to regulate the quantity of air entering the compressor, .a Venturi system to which the compressor delivers, means responsive to the absolute delivery pressure voi the compressor, means responsive to the diierential pressure between the inlet and throat of the Venturi system and means controlled :by said pressure-responsive means vto adjust the air-regulator means to vary the quantity of air passing through the 4compressor in vsuch a way that the ratio of the Venturi diilerential pressure to the compressor delivery pressure is maintained constant.

2. A control system for regulating the output of a centrifugal or axial flow compressor comprising a throttle valve on the intake side of the compressor, a Venturi system on the output side of the compressor, a capsule responsible to the absolute delivery pressure of the compressor, a capsule responsive both to the pressure at the inlet and at the throat of the venturi, a servosystem for adjusting the throttle valve and means applying the capsule adjustments to the servo-system whereby the throttle valve is adjusted to vary the quantity of air passing through the compressor in such a way that the ratio of the Venturi diierential pressure to the compresser delivery pressure is maintained constant.

3. A control system as claimed in claim 2 in which the adjustments of one capsule due to variations in pressure are applied to move the other capsule and adjustments oi the latter due to the first capsule and to variations in pressure are applied to the servo system.

A control system as claimed in claim 3 in which the capsules lie on opposite sides of, and are coupled to opposite ends of, a first-class lever, the end of one capsule remote from the lever being xed and the end of the other capsule remote from the lever being connected to the servo system.

5. A control system as claimed in claim 3 in which the fixed end of the rst capsule is manually adjusta-ble to move the capsule and rock the lever.

6. A control system asl claimed in claim 2, wherein the servo system opens, over a rst part of its range, the throttle valve to its full extent and over a further part of its range, it opens a valve on the delivery side of the compressor whereby a further rise of delivery pressure at the existing compressor speed is opposed.

7. A control system as claimed in claim 6 in which air is delivered by the compressor to an aircraft cabin and there is a refrigerator unit run by the compressed air to cool the air prior to its entering the cabin, a by-pass for the air being provided around the refrigerator unit with a valve to regulate the quantity of air passing through and around the unit, said by-pass valve being opened over the further part of the range of the servo system whereby after full opening of the throttle valve a further rise of delivery pressure at the existing compressor speed is opposed.

8. A control system as claimed in claim 6 in which a spill valve is provided to discharge air from the compressor to atmosphere, said valve being opened over the further part of the range of the servo system whereby after full opening of the throttle valve a further rise of delivery pressure at the existing ycompressor Aspeed is op posed.

9. A control system as claimed in claim '7 wherein the lservo system rst opens the refrigerator by-pass valve and vthen the spill valve.

10. A control system as kclaimed in claim '7 wherein the refrigerator by-pass `valve is normally regulated by a temperature control in the cabin and the servo system over-rides said control Vto open the lby-pass valve when, with a fully open throttle, 4the pressure requirements cannot lbe met.

'11. A rcontrol system as claimed in claim '7, wherein there is means to by-pass air around the refrigerator'unit when the aircraft is flying'above a predetermined altitude.

12. A control system as claimed in claim 11 in which there is an additional by-pass to convey 'air around the refrigerator unit, said by-pass having a valve which is regulated by a barometric capsule to open during ascent at the predetermined altitude.

13. A control system as claimed in claim 8, in which the spill valve is manually adjustable independently of the servo system.

le. A control system as claimed in claim 2 wherein air is delivered by the compressor to an aircraft cabin and the compressor is driven through variable speed gearing, changes in speed ratio being solely dependent on altitude.

l5. A control system as claimed in claim 14 in which a given speed ratio is engaged at a certain altitude during ascent of the aircraft and is disengaged at a lower altitude during descent of the aircraft.

16. A control system as claimed in claim 2 wherein the servo system comprises a reciprocating servo-valve under the control of the capsules, a rotary vane master servo-motor regulated by the servo-valve, a rotary vane servomotor to adjust the throttle valve and a rotary valve of which the vane of the throttle motor forms one element and of which the other element is adjusted by the master motor, `opening of the rotary valve on operation of the master motor to a certain extent bringing the throttle motor into operation to adjust the throttle and close the rotary valve when the throttle motor has been operated to a corresponding extent.

17. A control system as claimed in claim 15 in which the range of operation of the master motor is greater than the range of operation of the throttle motor and there is a second rotary valve which is adjustable by the master motor to operate a rotary-vane servomotor which adjusts the refrigerator by-pass valve, a lost motion device in the drive from the master motor to the second rotary valve ensuring that the refrigerator servomotor is brought into operation by the master motor only after the throttle valve is fully open.

18. A control system as claimed in claim 17 wherein the rotary valve of the refrigerator servomotor is normally adjustable by an actuator, the operation of which is regulated by said temperature control.

19. A control system as claimed in claim 17 in which the refrigerator servomotor is adapted to adjust the spill valve through the agency of a cam mechanism.

20. A compression plant comprising a casing, a driven rotary impeller therein, control means comprising a ow control device for varying the pressure of the working medium at the inlet to the impeller, means comprising a flow metering device positioned in the path of the working medium and responsive to changes in volume 110W through the impeller, and means comprising a contracting mechanism responsive to changes in the pressure of the Working medium at the outlet from the impeller, said flow control device being actuated by said second and third means to vary the inlet pressure in a manner such that the algebraic sum of a control action derived from the second means and of a control action derived from the third means is maintained equal to zero, said rst control action being in the sense to keep the volume ow constant and said second control action being in the sense to vary the volume flow in the same sense as the changes in outlet pressure.

21. A compressor plant as claimed in claim 20 in which the second control action is derived from means responsive to changes in the total head pressure at the outlet of the impeller.

BERNARD SIDNEY MASSEY. NEVILLE RAYMOND LLOYD QUINN. J OHN RAPHAEL ALLEN. STANLEY CLIFFORD WARNER.

REFERENCES CITED The following references are of record in the l0 file of this patent:

UNITED STATES PATENTS Number Name Date 2,284,984 Nixon June 2, 1942 2,353,201 Talbot July 11, 1944 2,385,664 Warner Sept. 25, 1945 2,479,991 Wood Aug. 23, 1949 2,539,430 Jepson Jan. 30, 1951 

